first off...congrats on your new STi. for your speakers they are 6.5 inch in the front doors and 4 inches in the back doors. u might have to put some spacers in the front if u have a speaker with a bigger magnet. as for the head unit, the 6 disc is cool for now, but if u want to upgrade later on, i would recommend an alpine or pioneer. i know a lot of other ppl will say alpine or pioneer too and some will say others like kenwood or other brands. its all in what youre looking for. tell us what kind of sound youre lookin for (and not just "i what it loud") i like that the fact the 04 STi doesnt come with a stereo as a standard. subaru understands that many of their customers will just upgrade their systems anyways.